1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, wherein an air-fuel ratio of an air-fuel mixture is feedback controlled to a given value based on a signal from an exhaust gas sensor which monitors a concentration of a certain component contained in the exhaust gas discharged from engine cylinders. The present invention particularly relates to the air-fuel ratio control system for such an engine as a V-type engine, which has an exhaust system divided into two lines for respective cylinder banks.
2. Description of the Prior Art
The air-fuel ratio control system for the V-type engine is known, as disclosed, such as, in Japanese Second (examined) Patent Publication No. 3-38417. The V-type engine has two cylinder banks and two exhaust passages connected to the respective cylinder banks. The air-fuel ratio control is performed by controlling air-fuel ratios of air-fuel mixtures for the respective cylinder banks, that is, output values of air-fuel ratio sensors for the respective cylinder banks, to be in antiphase or opposite phase with each other, that is, symmetrical with respect to a reference value. This symmetrical control of the air-fuel ratios is performed for purpose of preventing the torque fluctuation of the engine and the lowering of purification factors of catalytic converters provided in the respective exhaust passages.
On the other hand, following the tightening of automotive emission regulation, the so-called two-sensor system has been recently available, wherein air-fuel ratio sensors are provided both upstream and downstream of a catalytic converter. In this system, a deviation or an offset of a controlled air-fuel ratio relative to a window of the catalytic converter is detected based on an output of the air-fuel ratio sensor downstream of the catalytic converter for finely adjusting the controlled air-fuel ratio so as to eliminate such a deviation.
In the former conventional air-fuel ratio control system which performs the antiphase control of the outputs of the respective air-fuel ratio sensors, it is unknown how exhaust gases discharged from the respective cylinder banks are actually purified by the catalytic converters. On the other hand, in the latter conventional air-fuel ratio control system of the two-sensor type, due to a large transfer delay of the exhaust gas caused by the catalytic converter, an air-fuel ratio as monitored based on the exhaust gas downstream of the catalytic converter can not be controlled to the stoichiometric value (.lambda.=1), leading to large alternate deviations to lean and rich sides with respect to the stoichiometric value (.lambda.=1). This results in alternate emissions of harmful components, that is, NOx on the lean side and HC and CO on the rich side, to the atmosphere via a tail pipe.
For further purification of the exhaust gas, a catalytic converter may be further provided in a common exhaust pipe where the exhaust passages from the respective cylinder banks join each other at their downstream ends. However, in case of the V-type engine, when the exhaust gases discharged from the respective cylinder banks are in phase with each other in terms of air-fuel ratio, the harmful components are likely to be discharged via the tail pipe as exceeding the purification capability of the catalytic converter provided in the common exhaust pipe.
On the other hand, when the exhaust gases in antiphase with each other in terms of air-fuel ratio are introduced through the respective exhaust passages, the catalytic converter in the common exhaust pipe is effectively supplied with the mutually reactive components contained in the antiphase exhaust gases so as to achieve the purification thereof to a sufficient level.